The long-term goals of this research are to determine the effects of magnetic cross-relaxation between mobile and immobile protons on MRI images and to use magnetic cross-relaxation to obtain information about the solid components of tissue not present in conventional magnetic resonance images. To accomplish these tasks we propose to: 1. Determine the longitudinal, transverse, and cross-relaxation rates of the mobile and immobile protons with cross-relaxation spectroscopy in model, heterogeneous systems. 2. Construct a nuclear magnetic relaxation dispersion (NMRD) spectrometer to measure the magnetic field dependence of the proton T1 in model systems. 3. Measure cross-relaxation spectra in vivo in volunteers. Cross-linked protein and lamellar liquid crystal samples will be fabricated with different water contents and different ratios of H2O to D2O. The steady-state and transient effects of off-resonance RF saturation on the mobile proton magnetization of these samples will be recorded at 2T and 7T as a function of RF power. The longitudinal, transverse, and cross-relaxation rates of the mobile and immobile protons at both field strengths will be determined by fitting theoretical models to the collected data. A NMRD spectrometer will be constructed to provide an independent measure of the effects of cross-relaxation in the model systems. The combination of NMRD and cross-relaxation spectroscopy at 2T and 7T will determine the magnetic field dependence of magnetic cross-relaxation. The proposed NMRD spectrometer is easy to build and may be constructed in any laboratory with a NMR imaging system. Widespread use of NMRD could have important implications for biomedical research, e.g., decreased development time of MRI contrast agents. Pulse sequences currently used to create "magnetization transfer contrast" (MTC) images will be adapted to generate broad band cross- relaxation spectra in vivo on the GE l.5T Signa. These spectra will be used to determine the power, frequency, and duration of the saturation RF field which will maximize indirect saturation, minimize direct saturation, and maintain the RF SAR within FDA limits. The in vivo cross-relaxation spectra will be interpreted in light of the results from model systems.